The function and regulation of the herpes simplex virus type 2 (HSV-2) latency-associated transcript (LAT) was studied in vivo and in vitro. A. LAT regulation. Promoter sequences directing the transcription of the HSV-2 LAT were identified in transient expression assays and functionally compared in tissue culture with those for the HSV-1 LAT. The HSV-2 LAT promoter is stronger than the corresponding sequences from HSV-1 in all cell lines tested, but the relative activity of the two promoters varies among cell lines. Both HSV-1 and HSV-2 LAT promoters respond to inducers of cAMP and to HSV immediate-early gene ICP0. B. LAT function. A mutant of HSV-2 lacking the promoter sequences for the LAT (and thus unable to produce the LAT) and a rescued virus in which these sequences were restored were constructed. Characteristics of acute and latent infection in tissue culture and in guinea pigs were compared among the wild-type, mutant, and rescued viruses. During acute infections, the three viruses exhibited no appreciable difference in kinetics of replication or severity of lesions. Deletion of the LAT promoter sequences led to a marked decrease in spontaneous recurrences in guinea pigs latently infected with the mutant virus as compared with those infected with wild-type or rescued virus. Thus, the LAT gene plays a role in spontaneous reactivation from HSV-2 latency. The significance of this project lies in the identification of viral mechanisms important in establishment and reactivation from latency. An understanding of these mechanisms is critical for a complete evaluation of biological products which may influence the latent state.